The present application relates generally to the field of lithium-ion batteries or cells. More particularly, the present application relates to improved methods for initially charging (i.e., forming) such batteries.
Lithium-ion batteries or cells include one or more positive electrodes, one or more negative electrodes, and an electrolyte provided within a case or housing. Separators made from a porous polymer or other suitable material may also be provided intermediate or between the positive and negative electrodes to prevent direct contact between adjacent electrodes. The positive electrode includes a current collector having an active material provided thereon, and the negative electrode includes a current collector having an active material provided thereon. The active materials for the positive and negative electrodes may be provided on one or both sides of the current collectors.
FIG. 1 shows a schematic representation of a portion of a lithium-ion battery 10 such as that described above. The battery 10 includes a positive electrode 20 that includes a positive current collector 22 and a positive active material 24, a negative electrode 30 that includes a negative current collector 32 and a negative active material 34, an electrolyte material 40, and a separator (e.g., a polymeric microporous separator, not shown) provided intermediate or between the positive electrode 20 and the negative electrode 30. The electrodes 20, 30 may be provided as relatively flat or planar plates or may be wrapped or wound in a spiral or other configuration (e.g., an oval configuration). The electrode may also be provided in a folded configuration.
During charging and discharging of the battery 10, lithium ions move between the positive electrode 20 and the negative electrode 30. For example, when the battery 10 is discharged, lithium ions flow from the negative electrode 30 to the positive electrode 20. In contrast, when the battery 10 is charged, lithium ions flow from the positive electrode 20 to the negative electrode 30.
Once assembly of the battery is complete, an initial charging operation (referred to as a “formation process”) may be performed. During this process, a stable solid-electrolyte-inter-phase (SEI) layer is formed at the negative electrode and also possibly at the positive electrode. These SEI layers act to passivate the electrode-electrolyte interfaces as well as to prevent side-reactions thereafter.
One issue associated with conventional lithium-ion batteries relates to the ability of the batteries to withstand repeated charge cycling that involves discharges to near-zero-volt conditions (so-called “deep discharge” conditions). This deep discharge cycling may decrease the attainable full charge capacity of the batteries, which is known in the art as capacity fade. For example, a battery that initially is charged to 2.8 volts (V) may experience capacity fade with repeated deep discharge cycling such that after 150 cycles the full charge capacity of the battery is much less than the initial capacity.
One consequence of capacity fade in rechargeable batteries is that the batteries will require increasingly frequent charging as the capacity fade progresses. In certain circumstances, this may be relatively inconvenient for the user of the batteries. For example, certain implantable medical devices may utilize rechargeable batteries as a power source. Increasing capacity fade will require the patient to more frequently charge the rechargeable batteries.
Accordingly, it would be advantageous to provide a rechargeable battery (e.g., a lithium-ion battery) with increased resistance to capacity fade for batteries that experience repeated deep discharge cycling.